There was no evidence to back up the rumour, an AFP fact-check concluded. But posts on social media claiming that Nigeria - the most populous country in Africa, and the continent's largest economy - had come under the control of an impostor were viewed more than 500,000 times.
The earliest online mention of the rumour identified by AFP was in a Twitter post from September 2017, in which a user wrote, "The Man Who Parades himself as 'Buhari' Is Not The Real Buhari. Is Jubril From Sudan."
With the text came a video of Nnamdi Kanu, the leader of the separatist Indigenous People of Biafra, telling supporters that the country's elected leader had died. Kanu has repeated the claim, each time without evidence, on his pirate station, Radio Biafra. He has labelled the purported look-alike "Jubril Al-Sudani."
According to AFP, the conspiracy is nurtured by ethnic prejudice against Buhari, who is a northern ethnic Fulani Muslim. He has been accused of ignoring attacks against Christian farmers. Meanwhile, the May murder of a Nigerian diplomat in the Sudanese capital of Khartoum became the seed of the bizarre geopolitical suspicion feeding the rumour.
Asked about the impostor theory in the appearance yesterday, Buhari blamed his political opponents, saying, "A lot of people hoped that I was dead and hoped I died."
He said the false reports of his death even prompted some to contact his vice-president because they assumed he would replace him. "That embarrassed him a lot because he visited me when I was in London convalescing," Buhari told his audience in Poland.
The president, who was elected 2015, pinned a video of his response to the top of his Twitter page, where he also wrote: "One of the questions that came up today in my meeting with Nigerians in Poland was on the issue of whether I've been cloned or not. The ignorant rumours are not surprising - when I was away on medical vacation last year a lot of people hoped I was dead."